The reduced form of ascorbic acid in urine can falsely depress semiquantitative urine tests for glucose, hemoglobin, nitrite, etc. by reversing the oxidation of colored indicators. The reducing activity of reduced asorbic acid can also falsely elevate the apparent glucose when using the clinitest tablet method. Our studies show that ingestion of modest amounts of vitamin C supplements is sufficient to cause such interference in urinary glucose determinations. This project is designed to study the extent to which this is a practical problem by measuring reduced ascorbic acid in subjects taking vitamin C supplements and relating the ascorbic acid concentration to the extent of interference with methods in use in our laboratory.